Patent GB-1285896-A (GB)
Title: Electronic thermal devices for producing thermal patterns representing characters
Inventor: BRENNAN JAMES JR
IPC: B41J2/365
Filed: 1969-11-13
Abstract: 1285896 Automatic control of electric heating TEXAS INSTRUMENTS Inc 13 Nov 1969 [31 Dec 1968] 55679/69 Heading H2H In a thermographic high speed printing apparatus, in order to ensure uniform density of print when reproducing characters successively on thermochromatic material or thermally sensitive paper, the temperature of a matrix of character forming elements is sensed immediately prior to a print cycle and a value corresponding to the temp. sensed is stored in a capacitor and is used to control the electric heating power applied to the matrix during the succeeding print cycle thereby to produce a uniform maximum temperature. Thus account is taken of residual heat in the matrix and power is supplied sufficient only to make up the losses. In a modification the number of elements activated to form a character is also taken into account. As shown Fig. 2, a print head comprises a 5 Î 5 matrix of thermally isolated semiconductor mesas 12 bonded to a ceramic chip 14 by an epoxy resin layer. A transistor 18 and heating resistor R20 (Fig. 4) are diffused on the interior face of each mesa. A buffer transistor 22 is diffused on the face of a semiconductor chip 24 in the area defined by dotted lines 26. The ceramic chip is bonded to a metallic heat sink (28) Fig. 3, not shown. A temperature sensing diode 52 is located on the ceramic chip, the offset voltage across the diode being significant of the average temp. The temp. is sampled when a negative going pulse is applied at input 27, which results in conduction of a transistor switch 64 and the potential at a point 62, and which is significant of the temperature, is applied to capacitor 64. The charge in C64 is applied as one input to an amplifier 76 and is dependent upon the offset voltage across the diode 52. A second input to the amplifier 76 is pre-adjusted by a potentiometer 86 so that the voltage at its slider equals the voltage at point 62 when diode 52 is at ambient room temp. and is modified by a feed back current from the print head via R84 so that the amplifier output will be proportional to the stored voltage. As the temp. of the head increases due to increase in printing speed or due to the characters printed, the voltage drop across diode 52 decreases, decreasing the input voltage to 76. Thus the output voltage of the amplifier output stage 78, 80 in a line 82 and necessary to balance the amplifier is also reduced. The amplifier output is further reduced by current through R86 the value of which determines the reduction in output voltage for a given increase in the temperature of diode 52. Since the amplifier output controls the collector current of the print head transistors 18, 22, the electric heating power supplied to the head is reduced. In a modification, Figs. 5, 6 (not shown), the temp. sensor is a thermistor mounted on the back face of the heat sink. Also the amplitude of the charging voltage of the capacitor is modified in dependence upon the number of elements energized to form a character, the more the number of elements the greater the applied charging voltage. The capacitor is charged during a print cycle and is discharged during a non-printing cycle. The charge and discharge of the capacitor simulates the rate at which the matrix is heated and cooled relative to the temperature of the heat sink and compensates for thermal delay from the matrix to the thermistor. The voltage applied to the non- inverting input of the amplifier is a measure of the thermistor temperature and the charge on the capacitor estimates the temperature difference between the matrix and thermistor prior to the print cycle. The supply voltage and hence power applied to drive the matrix is then set at a level related to the absolute temp. of the matrix during the succeeding print cycle and uniform print density will be achieved under all conditions.